We propose to use this Program Project Grant as an opportunity to explore a diverse set of important biological issues. The topics fit in well with the collaborative theme of this PPG and will allow my laboratory to explore several new, important avenues of structural virology. A major theme of this grant is to use structural biology methods to examine the processes involved in the vector transmission of plant viruses. Using cucumber mosaic virus (CMV) as a model for viruses that are transmitted via aphids without helper proteins, we plan to examine the structural motifs that interact with aphid mouthparts and are required for aphid transmission. Preliminary work has identified surface residues for aphid transmission. We propose to extend the resolution of the current CMV model. This, combined with immunological and cryo-TEM techniques, will allow use to examine aphid-virus interactions. With poty viruses, a helper protein, HC-Pro, is required to be transmitted by aphids. The N-terminal domain of HC-Pro is cysteine rich and is responsible for vector transmission and systemic spread throughout the plant. As a third approach to understanding transmission of plant viruses, we are pursuing structural studies on the carmovirus, cowpea mottle virus (CPMoV). We have determined the low resolution (approximately 23 angstroms) structure of CPMoV, and the capsid organization is very similar to that of another member of the Tombusviridae tomato bushy stunt virus (TBSV). However, unlike TBSV, CPMoV is not sensitive to RNAse- a property thought to be essential for its transmission by beetles. Structural details of this virus should elucidate the reason underlying these differences in RNAse sensitivity. We will initiate crystallographic studies on the non-structural proteins of the flaviviruses. While this work is just beginning, the PPG will facilitate collaborative efforts on these medically important viruses.